Your hearing is definitely better than most, Mike, if you can hear a <1dBA difference (granted, that's weighted, but still). The "threshold of hearing" for delta is just over 1dB for most people...in reality they've all damaged their ears so much it's closer to 2 or even 3dB.

And I prefer to run my 8k RPM fans at the speed of "smashed by hammer"

I just used the 8k screamer to demonstrate the speed curve, I wouldn't run it either. I only have one fan in my entire case and it runs at 700 rpm. By a blind A/B test I don't mean asking a bunch of people "can you hear it?". Set up sample boxes with one fan at 12v and 2 at 6v, then you say which is louder, this or this? If A is quieter, people with good hearing will say A, people with average hearing will say A, and people with bad hearing will say "what?".

Another thing to note is the current that these fans draw at 12 and 6 volts. My guess is that the fan will draw half the current at 6v (you can easily test this). This amounts to 25% power and although RPM and CFM remain at 50% (I was wrong on the RPM of this fan) something is lost.

Just as in an electrical system where volts are useless without amps or in a mechanical system where rpm is useless without torque, with fans, CFM is useless without the pressure to back it up. Since we're just blowing air through a case, we don't need extreme pressures, but cases that use ducted CPU exhausts or other restrictive passages need more pressure than others.

I'm sure you know that when you put two fans in parellel their CFM is (roughly) doubled.
When you couple fans, their static pressure is doubled.

Just as you noted, the CFM of a fan goes up somewhat linearly with the voltage. The same is true with static pressure. A fan's air moving ability is somewhat a product of these two numbers.

Assuming a 6v fan pulls half the current of a 12v fan:

Running the 2 fans at 6v probably pulls half the power of one fan at 12v. Doesn't it seem strange that 2 6v fans each moving 20cfm consume half the wattage of a single 12v fan moving 40cfm? The catch is that you're now running at half the static pressure which will translate to a much lower CFM once the fans are installed than if the 12v fan was installed.

My guess is that if you want the same air moving ability (CFM and pressure) as a 12v Nexus you'll need 4 fans running at 6v in a series/parellel configuration. Two side by side with the other two stacked on top. This will also probably give you the same power consumption as a single 12v Nexus.

Check the current draws of the Nexus fan at 12v and 6v. If they're different, that's a red flag that the air moving ability of 2x6v fans will probably not match the performance of a 1x12v fan in anything but a free air environment.

In an unloaded electrical environment, two 6v lantern batteries in series will equal the voltage of a car battery, but only one will start my car.

Running the 2 fans at 6v probably pulls half the power of one fan at 12v. Doesn't it seem strange that 2 6v fans each moving 20cfm consume half the wattage of a single 12v fan moving 40cfm? The catch is that you're now running at half the static pressure which will translate to a much lower CFM once the fans are installed than if the 12v fan was installed

Hum this is getting interesting. We need to get some empirical data on Nexus current draw vs. voltages. I'm to lazy/busy. Looking forward to other posts though.

...to bad we can just plug a probe at the inake and exhaust of a case and get an ohm value...

I know this thread isn't active at the moment but I just came across it. The 3db "rule" is valid and still should apply with two versus one fan. The 3db increase is occuring - when total acoustic energy is concerned. MikeC's SLM didn't show it - but that doesn't mean the 3db increase didn't happen. It is much more likely to be an issue of acoustic cancellation. Two fans measured by a single microphone, especially very close to the fans, will be subject to narrow bands of cancellation. The fans also generate sound in multiple directions. Mike's SLM picked up primarily the sound from only one side of the fans. The sound from the other side presumably went "away from" the mic and eventually was absorbed in the room. These effects are very well understood by loudspeaker designers - you see the impact in commercial speaker systems. You rarely see two tweeters right next to each other precisely because of radiation patterns. When multiple tweeters are used in a row or a line array it is done to take advantage of the radiation patterns. It isn't a problem with woofers because the wavelengths are long. The proper way to measure this might be in a reverberent room, not anechoic, so the microphone could capture nearly all the radiated sound of the fans. The mic should also be further away to enable proper blending of the sound. I realize that is very difficult - and the close placement of the mic to the fans is necessary because of the very low noise levels.

Another thing to note is the current that these fans draw at 12 and 6 volts. My guess is that the fan will draw half the current at 6v (you can easily test this). This amounts to 25% power and although RPM and CFM remain at 50% (I was wrong on the RPM of this fan) something is lost.

When you say "something is lost", I would say "something is saved", basically you save energy because you get roughly the same airflow and consume less electricity.It might sound too good to be true, but here is my explanation:the electrical power that a fan consumes is not directly transformed into pressure & airflow. There is a loss.The output power of a fan is airflow multiplied by pressure (airflow in cubic meters per second, pressure in pascals).For example, if a fan runs at full speed in a completely open environment, the static pressure it creates is 0, therefore it doesn't give any power at all, but still consumes electricity. In this extreme case, efficienty is 0%In a computer case, static pressure is relatively small in comparison to what the fan is capable to do, so we are nearly in the same case: fan efficiency is very low.Let's say that it is 5%, for example( this is arbitrary ). Using 2 fans in parallel at half the voltage might increase efficiency to 10%.But you can't keep multiplying fans and reducing their voltage like that forever, because at some point the fans will be so weak that the static pressure of the case will be relatively high for them, and then it's a different problem.

Solid Snake wrote:

Just as in an electrical system where volts are useless without amps or in a mechanical system where rpm is useless without torque, with fans, CFM is useless without the pressure to back it up. Since we're just blowing air through a case, we don't need extreme pressures, but cases that use ducted CPU exhausts or other restrictive passages need more pressure than others.

I'm sure you know that when you put two fans in parellel their CFM is (roughly) doubled. When you couple fans, their static pressure is doubled..

I don't agree. In a computer case, mounting 2 fans in series will not double the static pressure.In a system with a very high impedance, then yes, it would (roughly).You can't choose both the pressure and airflow. They depend on each other. For a given pressure, the case will determine what airflow it allows. For a given airflow, the case will determine what the pressure needs to be.

Solid Snake wrote:

Just as you noted, the CFM of a fan goes up somewhat linearly with the voltage. The same is true with static pressure. A fan's air moving ability is somewhat a product of these two numbers.

Assuming a 6v fan pulls half the current of a 12v fan:Running the 2 fans at 6v probably pulls half the power of one fan at 12v. Doesn't it seem strange that 2 6v fans each moving 20cfm consume half the wattage of a single 12v fan moving 40cfm? The catch is that you're now running at half the static pressure which will translate to a much lower CFM once the fans are installed than if the 12v fan was installed.

My guess is that if you want the same air moving ability (CFM and pressure) as a 12v Nexus you'll need 4 fans running at 6v in a series/parellel configuration. Two side by side with the other two stacked on top. This will also probably give you the same power consumption as a single 12v Nexus.

You can't control both airflow and pressure indepandently.Have a look at this page:www.adda.com.tw/engineering/info5-2-1-s.htmOn the "Fan Curve" graph, the red "System Resistance Curve " shows you that for 1 pressure value, there is only 1 possible airflow value, and vice versa.

Solid Snake wrote:

Check the current draws of the Nexus fan at 12v and 6v. If they're different, that's a red flag that the air moving ability of 2x6v fans will probably not match the performance of a 1x12v fan in anything but a free air environment.

On that same graph there is an efficiency curve. You can see how it varies... it is near 0 in the "free flow" case, at the far right of the graphic. Installing fans in parallel will help get closer to optimum efficiency, so it is not surprising that the power consumption decreases.
Electrical power consumed is not equal to the power transfered to the airflow.

On that same page, there is a graph called "System Flow Rate Adding ", which shows you the benefits of 2 fans in parallel or in series.
On that graph, the system has a high impedance, so the "series" option ( in red ) is preferable to the "parallel" one (in blue) . In PC cases, it's usually the opposite: impedance is relatively low.

I found a page that explains it clearer :
http://www.users.dircon.co.uk/~nmb/fans/engineer/data/multiple.htmIt shows why in our situation ( "low system impedance"), there is a big improvement when using fans in parallel, and nearly no improvement when using them in series.
And it's the other way round for systems with high impedance.

MikeC's SLM didn't show it - but that doesn't mean the 3db increase didn't happen.

As I already mentioned above, it was because of the A weighting that the 3 dB increase did not show up.

Quote:

It is much more likely to be an issue of acoustic cancellation. Two fans measured by a single microphone, especially very close to the fans, will be subject to narrow bands of cancellation. The fans also generate sound in multiple directions. Mike's SLM picked up primarily the sound from only one side of the fans. The sound from the other side presumably went "away from" the mic and eventually was absorbed in the room... multiple tweeters ...

Not true. No such effects heard nor measured. The SLM was moved around. Not the same as tweeters at all, which typically operate >2kHz. Much fan noise is not that directional, especially at slow speed where it's mostly ~<300Hz noise that dominates.

I've intended to post about this, but here's an excellent thread to add to.

First, about halving the c.f.m. by using two fansMike C said that two fans @ 6V move as much air as one fan @ 12V - not fully true. Check the above diagrams from Adda, curve c in the second picture. With one fan, intersection with c-curve is at "19", whilst 2 fans intersect at "24" (my measurements). Now, for fans c.f.m. will follow rpm linearly, and pressure 'to the square', i.e. the same as the impedance in a PC case. Therefore, reving down the two fans will follow the c curve, going from 24 to 19, or from 12V to 9.5V. Now we have the situation of two fans giving the same c.f.m. as one fan at higher rpm. (What you see is impedance making two fans less efficient - for higher and higher impedance this inefficiency will increase as you can see on the b curve, where c.f.m. increase is very small).
OK, over to noise for this: On fan @ 12V generates X dB. Two fans will add 3 dB, two fans revved down as above will give 50xlog(19/24)=5 dB decrease, a total of 2 dB decrease. This means, Mike C has measured free flow figures, where the double cfm is real. But in real PC cases, you will find less benefit from two fans, depending on the impedance curve.

Secondly, are bigger fans better?I've used the same techniqe as above when comparing using two 80 mm fans vs. one 120 mm, taking the curves and SPL figures from www.papst.de. My results indicate that the 2x80 is better, or at least not worse than 1x120.
Example: 2x8412NGLE (80 mm) @ 12V generates 30 cfm @ 3.5 Pa, (12+3=15 dB). Extrapolate that curve to hit 1x4412FGL (120 mm) on 45 cfm @7 Pa (26 dB). Taking the 120 mm fan down to 30 cfm will decrease SPL by 50xlog(30/45)=9 dbB, totaling 17 dB. So, no benefit from using 120 mm fans, apart from price.

Please counter this, it seems to go against what's said elsewhere on the forum. I'm not here to flame, Mike C did a good job with his measurements, but for practical purposes I think you nedd to account for case impedance.

I'm not pretending to understand even half of what all you smart people have written in this thread. But I had this thought I'd like to share with you. By all means hack me to shreads if I'm wrong.

Concerning the one vs two fan debate:

One (~12V) fan has to accelerate air to double the speed of two (~6V) to push the same total cfm. Newtonian physics should do fine here. It says that speed squared = energy.

So two slow fans push same cfm at half the energy, because they do so at lower air speeds. So why are people saying wattage and not voltage should relate 1:1 to cfm? The single fan will heat up your room twice as much, is all?

is it better for me to use 1 fan @ 7v (yl d12sl) or 2 fans @ 5v (in terms of low dba).

Assuming these are all identical 120mm fans, 2 @ 5V.

NB. as you decrease voltage (and therefore RPM and CFM) motor noise becomes a bigger part of the overall noise and air turbulence less; so for fans with noisy motors 1 @ 7V is probably better, but the YL has a very quiet sleeve bearing design.

I think you need to know the total noise output of the fans to be certain. You will certainly end up with more CFM using 2 @ 5V rather than 1 @7V: just over 40% more in fact.

In dBA terms you have both a reduction and an increase, so the net effect is hard to calculate. Each fan at 5V will produce less noise than the 7V fan, but then you have to add the 3dBA back in because you have two fans.

One aspact of all this that we have not discussed in theis thread is the CFM to noise ralationship in any given fan.

As noise is a form of energy, then I'd expect (as a first order approximation) the noise produced by a fan to be capped by the square of it's rpm, or the square of it's input voltage. But then we have to consider the conversion to a log scale and compensation for human audio sensitivity.

Also, I suspect that as air velicity decreases the noise enrgy as a propoportion of the toatal energy imparted to the airflow (mV*2) decreases. I found the noise reductions that Mike C reported in his initial post stunning small - my subjective experience is that the reduction in noise between a fan at 6V compared to one at 12V is more like 10dBA than 3dBA.

I've done a few calculations and it does seem that a computer has very low pressure drop. A heatsink has an airflow area of 12x6 cm and 4 cm deep and with an airflow of 37 cfm there's a pressuredrop of 0.04 mmH2O. For example a gt silentblade 120mm fan has full air flow at 0.61 mmH2O. So no problem at all for that fan/heatsink combination. If you want double the airflow though it's not as easy since you now have to have 4 times the pressure.

But for cooling a case, pressure is no problem even when undervolted. I have long thought about using lots of silent fans in a case to see if it cools well and is silent. Since having running a fan at 5 volts gives a noise reduction of 18 db adding more fans quickly increases the airflow but not the db. Since I already have 3 fans that are inaudiable at 5v inside the case. Adding 3 more should still be inaudiable but provide alot more airflow. My "quest" is to have a very silent computer that's cool even when overclocked. Anything over 50 and I start getting worried...

I've done a few calculations and it does seem that a computer has very low pressure drop. --- If you want double the airflow though it's not as easy since you now have to have 4 times the pressure.

But for cooling a case, pressure is no problem even when undervolted. ---Since having running a fan at 5 volts gives a noise reduction of 18 db adding more fans quickly increases the airflow but not the db.

Since I already have 3 fans that are inaudiable at 5v inside the case. Adding 3 more should still be inaudiable but provide alot more airflow. -My "quest" is to have a very silent computer that's cool even when overclocked. Anything over 50 and I start getting worried...

note though that I used an online calculator and there's one value that --Guess that's about it for my ranting this time...

Thanks for reply JaRoD,

I Believe that:
In theory, there is no difference between theory and practice. But in practice, there is.

Then I don't use the calcolator from your links, because I don't believe in data sheets of a fan, but you give me some ideas:

1' Geometric shape for the Tunnel:
In theory my first idea is like that:
Make this an improvement ?
The next problem is cooling HDD at the base

2' dimension of a fan
bigger is better
I want to try in the Tunnel, a good 14cm fans, because theorically I think 14cm fan, is better than 12cm fan.
For my taste, I prefer sleeve bearings fans.
Can someone advise me about a very good 14cm very low dB fan ?

3' with low decibel, many fans cause a meaningful increase of motor noising and IMHO this is the problem:
[Â°C/W] / sec
I think cooling of a PC is a subjective compromise of that parameters.

Yeah, if there's anything at all interfering with the airflow the pressure builds up.

I'm actually using 14cm fans made by spire. 24db at 12v but that's too loud and if there's any resistance it just gets louder. But at 5v it's pretty ok as long as the case has damping and there's no direct soundpath to you and you need a pretty large hole in the case to keep the airspeed down too. I have one fan in the middle/bottom of the case sucking air through the hdds and it's surrounded by damping. Can't hear that one right now anyway. But the other one sucks air through the 3x5 1/4 drive bays and the air flow seems to make the most noise

And I've actually started thinking that going with lots of smaller fans actually makes more sense for cooling a case since there's no problem putting 2 80mm or 92mm fans on the width in a case and maybe even stack them 2 high for a total of 4 fans. Compared to only fitting one 120/140 mm fan. If you run the smaller ones at 5v you still get lots of airflow and they should be pretty silent too.

I almost did that today since I can get silentblade fans for 4ish euros each. But maybe I'll let someone else try it When I get typing it usually just flows...
[rantmode off]

ps. if you run a fan at 5v instead of 12v it handles 5.8 times less pressure than at 12v

yes you do get quite a bit of airflow, at 5v cfm is 42% of cfm at 12v and if you have 4 fans it's cfm*0.42*4 -> 1,68*cfm of one fan and and 12db less noise than 1 fan. Or atleast in theory... So with 4 25 cfm fans at 5v at 20 db you get 42cfm at 8db as the end result.

I want to try in the Tunnel, a good 14cm fans, because theorically I think 14cm fan, is better than 12cm fan.For my taste, I prefer sleeve bearings fans.Can someone advise me about a very good 14cm very low dB fan ?

The issue of multiple vs single fans is clear with the Nexus 120. Two of them at 6V move as much air as a single one at 12V, and make 2-3 dBA LESS NOISE!!

I don't think there's any way in hell this is true. CFM does not follow voltage, it follows WATTAGE. Anyone here familiar with Ohm's law knows that P = V^2/R.

I don't know what the impedance of these nexus fans are but it doesn't really matter. For this or any fan, running it at 6v will only consume (6^2/12^2) 1/4 of the power and probably run at 1/4th of the speed.

So what do we have here? 1 fan running at 100% speed vs 2 fans running at 25% speed. 3dB is the difference between doubling (or halving) sound output. If we can assume that RPM and sound output are related, then your "discovery" is far from ground breaking.

Let's say this Nexus fan runs at 1600RPM @ 12v. At 6v, this same fan would run closer to 400RPM NOT 800RPM like many would think. This is because of ohm's law. When you reduce voltage by 50%, you reduce power, noise and airflow by 75%. You've basically proven that two fans, together producing half the airflow are half as loud as one fan at full speed.

The 3dB reduction in sound you've discovered is right in line with the 50% airflow penalty.

I believe this rule applies only wtih pure resistance circuits and might not be the case here.

MikeC, can you clarify how you came to your 12V/6V airflow approx 2:1 result. Did you happen to use your newest fan testing methodology? That would help settle this dispute and satisfy my curiousity

Green Shoes wrote:

Yeah.....what we really need here is an anechoic chamber. That's the only way to get true measurements of these things.

I think MikeC's results are more applicable to our daily computing environment (Home office/Bedroom/Living Room), and I am more interested in results with realistic ambient noise. What this means is that I should probably put that top fan back on my P180 and undervolt those exhausts more .

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